1. Field of the Invention
The invention relates to the treatment of pistons to improve their resistance to detonation damage.
A conventional piston comprises a crown and a ring band, extending around the crown and meeting the crown at an edge. One or more piston ring grooves are formed in the ring band and carry respective rings; the uppermost of which is a compression ring sealing against an associated cylinder or liner to prevent the downward passage of high pressure gases from the combustion of a fuel/air mixture above the crown. The detonation of fuel can cause damage to the piston, particularly to those portions of the crown and the ring band adjacent the edge between them. This is an increasing problem, since in recent years engine designers have tended to design engines which operate with higher air/fuel ratios. In addition, less and less lead is being used in fuels. Both of these measures produce an increased tendency for an engine to `pink` or `knock` which, in turn, leads to an increased likelihood of detonation damage.
2. Review of the Prior Art
It has been proposed to protect those parts of the piston likely to be damaged by detonation by applying to those regions a coating which is more resistant to detonation than the material of the piston. For example, it has been proposed to apply a coating of nickel, by electroplating or by electroless plating of nickel over the whole area of the crown and around the ring band as far down as the top compression ring.
In operation, however, a piston oscillates in the associated cylinder or liner and also tends to pivot about a gudgeon pin, by which the piston is connected to an associated connecting rod. As a result of this, there is a tendency for the upper part of the ring band adjacent the edge between the ring band and the crown to touch the associated cylinder or liner. The presence of a coating, such as nickel, in this region can lead to scoring or scuffing on the associated cylinder or liner, and this is plainly undesirable. Under certain conditions, it is even possible for the coating to be scraped-off the piston and particles of coating in the lubricant are another source of unwanted scoring or scuffing.